What does USDA hardiness zone 5b mean in Maine?

Zone 5b means an average annual extreme minimum winter temperature of −15 to −10 °F — the standard USDA measure of which perennials survive winter. 24 locations in Maine fall in this zone. It does not set planting dates; see the frost calendar below for that.

Where in Maine's zone 5b is the growing season longest?

Machias runs the longest season on this page at about 185 days; Gorham is shortest at about 136 days — they share the same hardiness zone, but they sit at different weather stations, so their frost calendars differ.

When should I plant tomatoes in Maine's zone 5b?

Using Augusta's frost dates (this page's median-season location): start seeds indoors about Mar 7 – Mar 21, then transplant outside about May 9 – May 16. Your own location in the table above may differ by a week or two — use the ZIP planner on the home page for your exact station's dates.

Does every location in Maine's zone 5b have the same frost dates?

No — they do not all share a weather station, so frost dates vary by a week or more across the 24 locations listed (see the table above for each location's own dates).

USDA zone 5b in Maine

−15 to −10 °F avg. annual extreme minimum · 24 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 5b covers 24 locations across Maine (average annual extreme minimum −15 to −10 °F): Auburn, ME, Augusta, ME, Augusta, ME, Bath, ME, Belfast, ME, Belfast, ME, and 18 more. Average last spring frost across this group ranges April 21 (Machias) to May 16 (Sanford), and growing seasons run 136–185 days (Gorham to Machias) — even within one state, because hardiness sets winter survival, not planting dates. The table below gives every Maine location's own frost dates; the printable planting calendar uses Augusta, this group's median-season location.

USDA zone: 5b−15 to −10 °F
Last frost range: Apr 21–May 16avg, 32°F
First frost range: Sep 30–Oct 26avg, 32°F
Growing season range: 136–185days

Locations in this group

Every location below shares this zone and a nearby weather station, but keeps its own frost dates — find your town for its own numbers, or use the ZIP planner on the home page for an instant personalized answer.

Own frost dates per location.
Location	Also covers	Nearest station	Last frost	First frost	Season (days)
Auburn	Brunswick, Topsham, Lisbon Falls, Bowdoinham, Lisbon, Greene, +1 more	Durham	May 15	Oct 2	139
Augusta	Hallowell, Winthrop	Augusta State Ap	Apr 27	Oct 16	172
Augusta *	Winslow, Oakland, Fairfield	Waterville Trtmt Plt	May 2	Oct 10	159
Bath	Richmond, Wiscasset, Bowdoinham	Wiscasset Ap	May 8	Oct 7	151
Belfast	Waldoboro, Camden, Thomaston	W Rockport 1 Nnw	Apr 27	Oct 19	175
Belfast	Blue Hill, Searsport, Camden, Castine	Belfast	May 7	Oct 5	151
Calais	Eastport, Machias	Robbinston	May 5	Oct 13	160
Ellsworth	Bucksport	Bangor Intl Ap	May 4	Oct 4	152
Ellsworth	Blue Hill, Winter Harbor	Acadia Np	Apr 27	Oct 22	177
Gardiner	Richmond, Hallowell, Winthrop, Randolph	Gardiner	May 1	Oct 11	163
Gorham	Lake Arrowhead, Alfred, Standish, Sanbornville, Kezar Falls	Hollis	May 14	Sep 30	136
Machias	Milbridge	Jonesboro	May 3	Oct 9	158
Machias	Woodland	Woodland	Apr 21	Oct 26	185
Mechanic Falls	South Paris, Turner, Livermore Falls	Hartford	May 6	Oct 3	148
Mechanic Falls	Casco	Poland	May 3	Oct 7	157
North Windham	Cumberland Center, Gray, Freeport	Gray	Apr 28	Oct 19	172
Oxford	—	Bridgton 3 Nw	May 5	Oct 5	151
Saco	Biddeford, West Kennebunk	Kennebunkport	May 9	Oct 3	146
Saco	Westbrook, Gorham	Portland Intl Jetport	Apr 26	Oct 15	170
Sanford	Alfred, North Berwick, Sanbornville, Milton Mills, Milton, Union	Sanford 2 Nnw	May 16	Sep 30	136
Turner	—	Turner	May 1	Oct 10	162
Waldoboro	Newcastle	Newcastle	Apr 27	Oct 18	172
Woodland	—	Grand Lake Stream	May 4	Oct 10	157
Woodland	—	Wesley	May 2	Oct 15	164

* Augusta is this group's median-season location — the calendar below is computed from its data.

Data: NOAA NCEI U.S. Climate Normals 1991–2020 (public domain) and USDA Plant Hardiness Zone Map 2023. Planting windows synthesized from U.S. Cooperative Extension guides.

Planting calendar (Augusta, representative)

Computed from Augusta's average frost dates — the median-season location in this Mainezone 5b group. hatched = start seeds indoors, solid green = plant out, teal = a fall sowing, and the terracotta dot marks the estimated first harvest. Ranges are extension-guide planning guidance, not guarantees — your own location's dates may run a week or two earlier or later (see the table above).

Crop

JanFebMarAprMayJunJulAugSepOctNovDec

Tomato

Pepper

Cucumber

Summer squash / zucchini

Start indoors
Plant out
Fall sowing
First harvest

Planting windows for Augusta (Maine, zone 5b), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 7 – Mar 21	May 9 – May 16	Jul 8 – Jul 28	—	matures comfortably
Pepper	Very tender	Feb 21 – Mar 7	May 16 – May 23	Jul 15 – Aug 14	—	matures comfortably
Cucumber	Tender	Apr 4 – Apr 11	May 9 – May 16	Jun 28 – Jul 18	—	matures comfortably
Summer squash / zucchini	Tender	—	May 9 – May 16	Jun 23 – Jul 8	—	matures comfortably
Bush bean	Tender	—	May 9 – May 16	Jun 28 – Jul 8	Aug 11 – Aug 21	matures comfortably
Sweet corn	Tender	—	May 2 – May 16	Jul 1 – Jul 31	—	matures comfortably
Basil	Very tender	Mar 21 – Apr 4	May 9 – May 16	Jun 8 – Jun 23	—	matures comfortably
Lettuce	Half-hardy	Mar 21 – Apr 4	Apr 4 – Apr 18	May 19 – Jun 3	Jul 28 – Aug 12	matures comfortably
Pea	Hardy	—	Mar 21 – Apr 4	May 15 – May 30	Jul 18 – Aug 2	matures comfortably
Spinach	Hardy	—	Mar 21 – Apr 4	Apr 30 – May 10	Aug 7 – Aug 17	matures comfortably
Carrot	Half-hardy	—	Apr 11 – Apr 18	Jun 10 – Jun 30	Jul 8 – Jul 28	matures comfortably
Broccoli	Half-hardy	Mar 7 – Mar 21	Apr 4 – Apr 18	May 29 – Jun 18	Jul 13 – Aug 2	matures comfortably

Which crops fit this growing season

Using Augusta's own 159-day growing season (32°F, NOAA 1991–2020 median) against each crop's real days-to-maturity range: 12 of 12 crops mature here with comfortable margin. A different location in the table above with a longer or shorter season will get a different mix — the ZIP planner on the home page gives your own.

Frost & freeze dates (Augusta)

From NOAA's 1991–2020 Climate Normals at station USC00179151. The median (p50) is the average date; the 90%-safe column is the date the freeze has passed in about 9 years out of 10 — the conservative date to plant after or harvest before.

Freeze probabilities by temperature threshold (MM/DD, NOAA 1991–2020), Augusta.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	May 14	May 27	Oct 1	Oct 11	137
32°F (freeze)	May 2	May 14	Oct 10	Oct 27	159
28°F	Apr 22	May 3	Oct 24	Nov 7	183
24°F	Apr 8	Apr 21	Nov 4	Nov 19	208

Growing degree days

Growing degree days (GDD) accumulate warmth above a base temperature over the year — a better predictor of crop development than the calendar alone. This group's median GDD (base 50°F) across all 24 locations is 2,055; Augusta's own reading is shown below.

Annual growing degree days for Augusta (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	2,201	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	4,086	cool-season crops (brassicas, greens)

Hardiness zone 5b in Maine

Zone 5b means an average annual extreme minimum winter temperature of about −15 to −10 °F on the USDA's 2023 map — the number that tells you which perennials, shrubs, and trees reliably survive an average winter here. It does not set your planting dates, which come from the frost calendar above. Maine spans 8 hardiness zones in this dataset — see all Maine locations for the full range, including zones3b, 4a, 4b, 5a, 6a, 6b, 7a.

Explore zone 5b in other states at zone 5b, or see all USDA hardiness zones.

Frequently asked questions

What does USDA hardiness zone 5b mean in Maine?: Zone 5b means an average annual extreme minimum winter temperature of −15 to −10 °F — the standard USDA measure of which perennials survive winter. 24 locations in Maine fall in this zone. It does not set planting dates; see the frost calendar below for that.
Where in Maine's zone 5b is the growing season longest?: Machias runs the longest season on this page at about 185 days; Gorham is shortest at about 136 days — they share the same hardiness zone, but they sit at different weather stations, so their frost calendars differ.
When should I plant tomatoes in Maine's zone 5b?: Using Augusta's frost dates (this page's median-season location): start seeds indoors about Mar 7 – Mar 21, then transplant outside about May 9 – May 16. Your own location in the table above may differ by a week or two — use the ZIP planner on the home page for your exact station's dates.
Does every location in Maine's zone 5b have the same frost dates?: No — they do not all share a weather station, so frost dates vary by a week or more across the 24 locations listed (see the table above for each location's own dates).

Sources & method

Frost, freeze, growing-season, and growing-degree-day figures are NOAA NCEI U.S. Climate Normals 1991–2020, one reading per location's own nearest station (see the table above). The hardiness zone is the USDA Plant Hardiness Zone Map 2023, matched to each location's ZIP. Planting windows are computed by counting from the average last and first frost using per-crop offsets synthesized from U.S. Cooperative Extension guides — the full method and citations are on the methodology page.